The present invention relates to calibration of sweep generation circuits, and more particularly to calibration of a sweep signal generator by use of time intervals measured on repetitive sweep signals produced by the circuit.
Instruments which measure electronic signals employ sweep circuits to establish a range of values against which a quantity to be measured can be compared. In oscilloscopes, for example, the displacement of the visible cathode-ray trace on the screen of the oscilloscope is controlled by a horizontal sweep circuit comprising a ramp or sawtooth wave generator which provides a sweep signal to the horizontal deflection circuits of the CRT. Such circuits are well known in the art of oscilloscope design and engineering.
Calibration of sweep signal generating circuits is a continual and time-consuming process. As is known, such circuits comprise analog components having characteristics which are determined to various degrees of precision and which can be altered by long-term effects such as aging. Further, those components can also exhibit short-term alterations in operating values in response to variations in the ambient environment of the circuit. These changes in component characteristics can affect the characteristics of the waveforms generated by the circuit which can result in degradation of the measurements whose range of values depends upon the sweep signals.
In order to establish and maintain the accuracy of a measuring system employing a sweep generating circuit, the circuit is periodically calibrated by altering the characteristics of the waveforms it produces to correspond with the characteristics of a standard or calibrating signal. This is accomplished by the provision of circuit components having adjustably variable characteristics, which permit alteration of the transfer function of the circuit as required to make the produced waveform comply with the calibrating signal. For example, the horizontal sweep generation circuit of an oscilloscope can include potentiometers and adjustable capacitors which can be manually manipulated to vary selected characteristics of the sweep waveform.
Calibration of instruments such as oscilloscopes which utilize sweep circuits to provide visual displays requires visual interpretation of displayed signals, manual adjustment of variable components, and manual operation of the instrument during calibration. This can entail a great many mutually dependent steps, which increase the possibility of adjustment error due to faulty perception and judgment. Since an oscilloscope is characteristically provided with the capability of generating a multiplicity of variable time-base sweep waveforms, calibration must be performed at a number of selected sweep speeds, which multiplies the required calibration time.
An example of a calibration procedure which is required to maintain a high-performance oscilloscope in operating condition is found in the 456B Oscilloscope Instruction Manual, published by Tektronix, Inc., Beaverton, Ore.
While the aforementioned problems relating to calibration of sweep generation circuits used by measuring instruments are considerable from the standpoint of operating and maintaining the instrument, they also complicate instrument design and fabrication. For example, a timebase generation circuit for a high-performance oscilloscope requires the provision of manually operated switch circuits which comprise arrays of high-precision components, such as capacitors and resistors. High quality, precision components provide better initial accuracy and exhibit less sensitivity to long- and short-term effects and thus reduce the need for calibration. However, a result of selecting high precision components is that the cost of constructing the instrument goes up.